灌渠闭闸期闸前段流场特征及泥沙淤积速率分析

Analysis of flow field characteristics and sediment deposition rate in irrigation canal in front of sluice during gate closing

为研究灌渠闭闸期闸前段流场特征及泥沙淤积速率,基于南亚某河流待建引水明渠建立水流数学模型,并借助Flow-3D软件对高水位、中水位和低水位3种特征工况下,闭闸期闸前段流场进行模拟分析。依据盲肠水流泥沙运动的研究思路,提出闭闸期闸前明渠段泥沙淤积速率的估算方法,并结合同期明渠段悬移质泥沙模型试验资料,率定得出闸前段泥沙淤积速率求解公式。结果表明,闭闸期闸前段出现围绕竖轴近似呈椭圆型的回流现象,平面流速呈交替增减、水位呈中心低周界高的水力特征;泥沙淤积速率较大,灌渠将呈现累积性淤积特征。最终得出,闭闸期闸前明渠段与港池内盲肠水流运动相类似,闸前渠段淤积严重,引水明渠的口门初步设计方案有待充分论证的结论。

To analyze flow field characteristics and sediment deposition rate of irrigation canal in front of sluice during the gate closing, a generalized numerical model was set up according to the preliminary design scheme of the scheduling diversion channel on a river in South Asia. By the software Flow-3D, simulation analysis of the flow field characteristics of canal in front of sluice was carried out under 3 characteristic conditions（high, medium and low water level） during the gate closing. In addition, the intensity of circulating under characteristic conditions was extracted, which was denoted by average velocity of water particle on the lateral axis of recirculation. Referring to the movements of water and sediment in closed channel, an estimation method of accretion rate of canal in front of sluice was proposed, and corresponding formula was derived, in which the coefficient α comprehensively reflected geometric hydraulic conditions and turbulent diffusion intensity of sediment at entrance. Based on the data of contemporaneous suspended sediment model, the average value of the coefficient α was 0.02331. We adopted 0.0235 for the coefficient after calibration and calculated the sediment deposition rate in canal in front of sluice under 3 characteristic conditions. Hence, approximate elliptic circulation appeared around vertical axis in the canal in front of sluice during the gate closing, and planar flow velocity showed fluctuation. Water level elevation was low in center and high all around. Planar flow velocity of inferior wall was larger than that of superior wall. Water level elevation of the former was higher than that of the latter. In fact, water in recirculation zone exhibited three-dimensional and transient property. By the Flow-3D, radial circulation around the streamline of planar circulation was obvious, which promoted mass, momentum and energy exchange to become uniform between the upper and lower flow constantly. Therefore, vertical flow velocity in recirculation zone was in different directions but relatively uniform in value, which differed from logarithmic or exponential vertical distribution of flow velocity in common open channel. On the whole, planar circulation occupied the main position, and the radial circulation was secondary in the canal in front of sluice. Intensity of planar circulation increased obviously with the main flow velocity increasing, owing to the increasing of turbulent shear stress. The flow field of canal in front of sluice was similar to that of closed channel in cecum branch during the gate closing. Results on cecum circulating flow can be used in canal in front of sluice. The sediment deposition rate of the canal in front of sluice is large, so the irrigation canal will present the trend of cumulative deposition, which certainly will bring serious influences on the operation and management of irrigation canal system. The trumpet-shaped cross section of the entrance is extremely unreasonable. Preliminary design scheme of the entrance in water diversion canal needs sufficient demonstration. However, the calculation of sediment deposition rate in the canal in front of sluice provides evidence for siltation analysis of the project under other conditions. Geometric and hydraulic conditions at entrance are various in main, branch or lateral canal. Influencing factors on the coefficient α in calculation formula of sediment deposition rate are numerous. The results in this paper can offer the reference for analogous engineering design.